In typical wireless communication system, a digital baseband processor produces an information-bearing signal that is processed by a transmitter lineup before radiation of an associated radio frequency (RF) signal over the air interface. For example, FIG. 1 is a simplified block diagram of part of a conventional transmitter lineup 100, which includes a digital front end (DFE) 110, a digital-to-analog converter (DAC) 120, a modulator (MOD) 130, a power amplifier (PA) 140, a demodulator (DEMOD) 163, an analog-to-digital converter (ADC) 164, and an antenna 150.
Generally, the DFE 110 processes digital baseband signals for various channels to pre-condition the digital signals for amplification and wireless transmission. In some systems, the DFE 110 may include several sequential processing blocks, such as a digital up-converter (DUC) block 112, a crest factor reduction (CFR) block 114, and a digital pre-distortion (DPD) block 116. The DUC block 112 receives signals S1 to Sn (e.g., n=2 to 5) from a baseband processor (not illustrated), where each of signals S1 to Sn corresponds to a unique digital, baseband, information-bearing signal that is intended to be conveyed using a distinct carrier signal. DUC block 112 up-converts the input signals, S1 to Sn, to a higher sampling rate, and produces a common digital composite signal, S3. The CFR block 114 performs peak-to-average power ratio reduction on the digital composite signal, S3, thereby creating a clipped digital composite signal, S4. The DPD block 116 then performs a digital pre-distortion process on the clipped digital composite signal, S4, so as to generate a digitally-predistorted baseband signal, S5. The digital pre-distortion performed by DPD block 116 is intended to pre-compensate for signal distortion that will be applied to an RF version of the signal by the downstream PA 140.
The pre-distorted digital baseband signal, S5, is converted to an analog baseband signal, S6, by DAC 120, and then upconverted by a modulator 130 to RF frequencies before being supplied to the PA 140. The PA 140 amplifies the analog RF signal, S6, resulting in a high-power RF signal, S7, which is produced at the output 142 of the PA 140. The high-power RF signal, S7, is then supplied to a system antenna 150, which radiates the signal over the air interface.
As indicated previously, the PA 140 may undesirably distort the RF signal during the amplification process, and the DPD block 116 is designed to pre-compensate for that distortion by pre-distorting the signal in the digital domain in an inverse manner. In order for the DPD block 116 to accurately pre-distort the digital signal, the DPD block 116 analyzes a feedback signal, SF, which is generated based on the amplified output signal, S7, that is traveling between the PA output 142 and the antenna 150. For example, a directional coupler 160 in proximity to a transmission line (e.g., a quarter-wave output transformer) between the PA output 142 and the antenna 150 may be used to generate the feedback signal, SF, which is a reduced-power (lower-amplitude) version of the amplified output signal, S7. The feedback signal, SF, is converted, along a feedback path 162, into a baseband signal by a demodulator 163, and the baseband feedback signal is then converted into a baseband digital signal by ADC 164 before being supplied to the DPD block 116.
In such a configuration, there may be an undesirably high level of insertion loss associated with both the quarter-wave output transformer and the directional coupler 160. As operational frequencies for wireless communication systems continue to increase, the detrimental impacts of these and other losses are becoming more acute. In order to achieve high efficiency, designers of wireless communication systems strive to reduce such losses in advanced transmitter and transceiver lineups.